To understand how the visual system in the monkey analyzes, classifies, and stores visual stimuli, we have been recording the responses of single neurons in two of the stations known to be critical for those functions, namely, striate cortex and inferior temporal cortex, while the animals perform behavioral tasks. We have found that attention to the stimulus to detect is dimming causes a weakened response to the stimulus by inferior temporal neurons, whereas attention to the same stimulus in order to discriminate it from another causes an increased neuronal response. Other factors, such as the particular feature of the stimulus being attended to, e.g., its texture or pattern, and the exact timing of the motor response also modify the neuronal discharge. In both inferior temporal cortex and striate cortex the pattern of a stimulus appears to be encoded not only by the number of action potentials in the neuronal discharge but also by the temporal sequence of action potentials. The temporal sequences are very similar from one neuron to the next implying that the mechanisms for producing stimulus-evoked spike trains are limited, and so encoding must involve various combinations of a few fundamental sequences.